isenthalpic mixing of water and air

[Clifford Bradford]

I am modeling the isenthalpic mixing of a relatively small (2.5% by mass) of water into air. The main simplifying assumption is that the final mixture pressure is the same as the air pressure. I am trying to validate a calculation in another package that uses this assumption. I've modelled it 3 different ways with CoolProp in the attached spreadsheet. The basic problem is 100lb/s of air at 1000°F and 500psia has 2.5lb/s of water at 60°F and 550psia injected into it and the problem is to find the resultant .

1. First calculation method (Column B). The mixture is treated as an ideal mix of air and water at the mixture temperature (row 19) and 500psia so the air and water specific enthalpies are evaluated as separate fluids (air is evaluated as "Air"). Then the residual is the difference between the inlet and exit total enthalpy which is "Goal Seek"ed to zero by varying the mixture temperature.
2. Second method (Column E): same as the first except air is evaluated as "Air.mix".
3. Third method: same as the second except the final properties are calculated with the mix of air and water. The calculation of the mix mole fractions is in G1:J6

the first two methods are essentially treating the mixture as a mix of ideal gases and give results within 0.1°F while the 3rd method method gives a lower final temperature by about 1.5°F. I'm assuming this is due to the mixture effect and not due to different zero references for enthalpy. My thinking is that ideal gas mixture assumption (first two calculation methods) doesn't care if the zero reference conditions for air and water enthalpy are different. For the other package that I'm going to implement this calculation in I'm probably going to have to go this route as I don't have any control of the reference conditions

Could someone verify my calculation with a front end that allows the zero reference to be set?

[Ian Bell]

I'd be willing to bet this is a reference state issue. I think it is not handled properly in CoolProp, and I believe also in NIST REFPROP, though I have always found this quite mysterious.

My best understanding is that for constant pressure adiabatic mixing to work properly with multi-fluid models, you need to use absolute zero reference states for all fluids and that is not what is used by default in CoolProp (or REFPROP). I have to admit, I am not 100% sure about this though.

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[Clifford Bradford]

I'm not sure. I expect my 3rd result to be different from the first 2. In some regards it doesn't matter because the program I'm using this to validate can only do the ideal calculation. Also, that program has a choice of Fluid Properties but some of them also have issues with reference conditions as fluid compositions change. It makes it really hard to do calculations with reactions/mixing.

[Ian Bell]

Mixtures and reactions are always tricky. That is what I learned from 10 years working at NIST on mixture thermodynamics :)

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[Lynn McGuire]

Hi Clifford,

You can download Design II for Windows from my website
    https://www.winsim.com/
and validate your mixing question.  Reference States do not matter in our software.

BTW, air at 1,000 F is extremely supercritical.  The best calculations will use the GERG 2008 Equation of State.  The next best is our Peng Robinson Equation of State.

BTW2, unless you turn it off, we automatically convert air into its major constituents of 79% N2 and 21% O2.

If you want, email me offline at L...@winsim.com and I will send you a download link.  Then you will need to get a two week password from our website.

Shoot, I went ahead and ran it.  The mixed stream is
Peng-Robinson:  864.9 F
GERG 2008:  866.7 F

Sincerely,
Michael Lynn McGuire
President
WinSim Inc.

To view this discussion visit https://groups.google.com/d/msgid/coolprop-users/57330d8d-8e3c-4114-8f87-b604ad1672a5n%40googlegroups.com.

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Sincerely,
Michael Lynn McGuire, P.E.
President
WinSim Inc.
USA 281-545-9200 x102 office
USA 832-202-3918 cell
USA 281-545-8820 fax
l...@winsim.com
www.winsim.com

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[Clifford Bradford]

Michael,

thanks that's helpful.